The present invention relates to thermal plasma processing for use in processing a substrate by applying thermal plasma onto the substrate, or a plasma processing apparatus and a method thereof, which applies plasma derived from a reaction gas or plasma and the reaction gas flow simultaneously onto a substrate so that plasma processing such as low-temperature plasma processing is carried out thereon.
Conventionally, a semiconductor thin film, such as polycrystalline silicon (poly-Si), has been widely utilized for thin-film transistors (TFT: Thin Film Transistor) or solar batteries. In particular, the poly-Si TFT has high carrier mobility, and is characterized in that it can be formed onto a transparent insulating substrate, such as a glass substrate. By utilizing this characteristic, the poly-Si TFT is widely used, for example, as switching elements forming a pixel circuit for a liquid display apparatus, a liquid crystal projector, or an organic EL display apparatus, or as a circuit element for a driver for driving liquid crystal.
As a method for manufacturing a high-performance TFT on a glass substrate, in general, a manufacturing method referred to as “high-temperature processing” is proposed. Among the manufacturing processes for a TFT, a process in which a high temperature of about 1000° C. is used as the highest temperature in the process is generally referred to as “high-temperature processing”. The characteristics of the high-temperature processing are that a relatively high-quality polycrystalline silicon can be formed as a film by utilizing a solid-phase epitaxy of silicon, that a high-quality gate insulating layer can be obtained by thermal oxidation of silicon, and that a clean interface between polycrystalline silicon and a gate insulating layer can be obtained. By these characteristics, the high-temperature processing makes it possible to manufacture a high-performance TFT having high mobility and high reliability in a stable manner.
In contrast, since the high-temperature processing is a process in which a silicon film is crystallized by solid-phase epitaxy, heating treatment for a long period, such as 48 hours, at a temperature of about 600° C. is required. This process is requires an extremely long period, and in order to increase the throughput of the process, a large number of heating furnaces are inevitably required, resulting in an issue of difficulty in reducing costs. In addition, since quartz glass needs to be used as an insulating substrate having high heat resistance, the costs of substrates become higher, with the result that this method is not suitable for providing devices with large areas.
In contrast, a technique referred to as “low-temperature processing” has been proposed in which the highest temperature during the processes is lowered so that a poly-Si TFT is manufactured on an inexpensive glass substrate with a large surface area. Among manufacturing processes of TFTs, the process for manufacturing a poly-Si TFT on a relatively inexpensive heat-resistant glass substrate under a temperature environment having the highest temperature of about 600° C. or less is generally referred to as “low-temperature process”. In the low-temperature process, a laser crystallization technique that crystallizes a silicon film by using a pulse laser whose oscillation time is very short has been widely used. The laser crystallization refers to a technique in which such a characteristic that the silicon thin film is instantaneously fused by irradiating a silicon thin film on a substrate with pulse laser light having high output and this fused silicon thin film is crystallized during its solidification is utilized.
However, this laser crystallization technique has some major issues. One of the issues is a large number of trap levels that are localized inside a polysilicon film formed by the laser crystallization technique. Due to the existence of these trap levels, carriers that are to be originally shifted through an active layer upon application of a voltage are trapped, failing to devote to electric conduction, with the result that adverse effects, such as a reduction in the mobility of TFTs and an increase of the threshold voltage, are raised. Another issue is that by the limitation of the laser output, the size of a glass substrate is limited. In order to improve the throughput of the laser crystallization process, it is necessary to increase the area that can be crystallized in one time. However, since the current laser output has limitations, upon application of this crystallization technique to a large-size substrate of the seventh generation (1800 mm×2100 mm), a long period of time is required for crystallizing one substrate.
Moreover, as the laser crystallization technique, in general, a laser that is formed into a line pattern is used, and the crystallization is performed by scanning this laser. Since this line beam has limitations in laser output, it is shorter than the width of a substrate, and it is necessary to carry cut the laser scanning process several times in a divided manner in order to crystallize the entire surface of the substrate. This generates joint regions of the line beam within the substrate, resulting in double scanned regions. The crystalline characteristic in these regions is greatly different from that in the region crystallized by the scanning of one time. For this reason, the element characteristics of the two regions are greatly different from each other and become the main cause of device deviations. Lastly, since the laser crystallization device has a complicated device structure, and sine the costs of consumable parts are high, there is an issue in that the device costs and running costs are consequently high. As a result, TFTs using a polysilicon film crystallized by using a laser crystallization device cause elements having high production costs.
In order to overcome these issues of the limitation of the substrate size and high device cost's, a crystallization technique called “thermal plasma jet crystallization method” has been examined (for example, see “Crystallization of Si in Millisecond Time Domain Induced by Thermal Jet Irradiation” S. Higashi, H. Kaku, T. Okada, H. Murakami, and S. Miyazaki, Japanese Journal of Applied Physics, Vol. 45, No. 5B, (2006) pp. 4313-4320). This technique is briefly described in the following. When a tungsten (W) cathode and a water-cooled copper (Cu) anode are made to face each other, and a DC voltage is applied thereto, an arc discharge is generated between the electrodes. By allowing an argon gas to flow between these electrodes under the atmospheric pressure, thermal plasma is ejected from an ejection hole opened in the copper anode. The thermal plasma is thermally equilibrium plasma forming an ultra-high temperature heat source in which temperatures of ions, electrons, and neutral atoms are almost the same, with the temperatures thereof being set to about 10000 K. For this reason, the thermal plasma can easily heat an object to be heated to a high temperature, and by scanning a substrate on which an a-Si film is deposited in front of the ultra-high temperature thermal plasma at a high speed, the a-Si film can be crystallized.
In this manner, since the device structure is very simple, and since the crystallization process is carried out under the atmospheric pressure, it is not necessary to cover the device with an expensive member, such as a chamber, and device costs are expected to be extremely low. Moreover, since utilities required for the crystallization are only an argon gas, power, and cooling water, this system provides a crystallization technique also having inexpensive running costs.
FIG. 20 is a schematic view that describes the crystallization method of a semiconductor film using this thermal plasma.
In FIG. 20, a thermal plasma generation device 31 includes a cathode 32, and an anode 33 that is placed with a predetermined distance apart from the cathode 32 so as to face therewith. The cathode 32 is made of a conductor such as, for example, tungsten. The anode 33 is made of a conductor such as, for example, copper. Moreover, the anode 33 is formed into a hollow structure so as to be cooled with water being allowed to pass through the hollow portion. Furthermore, an ejection hole (nozzle) 34 is formed in the anode 33. Upon application of a direct current (DC) voltage between the cathode 32 and the anode 33, an arc discharge is generated therebetween. By allowing a gas such as an argon gas to flow between the cathode 32 and the anode 33 under the atmospheric pressure in this state, thermal plasma 35 can be ejected from the ejection hole 34. In this case, “thermal plasma” is the thermally equilibrium plasma forming an ultra-high temperature heat source in which temperatures of ions, electrons, and neutral atoms are almost the same, with the temperatures thereof being set to about 10000 K.
Such thermal plasma 35 can be utilized for heating treatment for use in crystallization of a semiconductor film.
More specifically, a semiconductor film 37 (for example, amorphous silicon film) is preliminary formed on a substrate 36, and the thermal plasma (thermal plasma jet) 35 is applied to the semiconductor film 37. At this time, the thermal plasma 35 is applied to the semiconductor film 37, while being relatively shifted along a first axis (lateral direction in an example of FIG. 20) in parallel with the surface of the semiconductor film 37. That is, the thermal plasma 35 is applied to the semiconductor film 37, while being scanned in the first axis direction. In this case, “being relatively shifted” means that the semiconductor film 37 (and a substrate 36 supporting the semiconductor film 37) and the thermal plasma 35 are relatively shifted, and includes both cases in which only one of these is shifted and in which both of them are shifted together. By carrying out such a scanning process of the thermal plasma 35, the semiconductor film 37 is heated by a high temperature of the thermal plasma 35 to obtain a crystallized semiconductor film 38 (polysilicon film in this example) (for example, see Japanese Unexamined Patent Publication No. 2008-53634).
FIG. 21 is a conceptual view showing a relationship between the depth from the uppermost surface and the temperature. As shown in FIG. 21, by shifting the thermal plasma 35 over the substrate 36 at a high speed, only the proximity of the surface of the substrate 36 can be treated at a high temperature. Since the heated region is quickly cooled after the thermal plasma 35 has passed, the temperature on the proximity of the surface becomes high only for a very short period of time.
The thermal plasma 33 is generally generated in dot regions. Since the thermal plasma 35 is maintained by thermions discharged from the cathode 32 and the thermions are more vigorously discharged at a position having high plasma density, a positive feedback is exerted so that the plasma density becomes increasingly higher. That is, the arc discharge occurs by concentrating on one point of the cathode so that the thermal plasma 35 is generated at the dot region.
In the case where an attempt is made so as to uniformly process a flat-shaped substrate, such as in the case of crystallization of a semiconductor film, a dot-shaped thermal plasma needs to be scanned over the entire substrate. In this case, so as to provide a process that can be carried out in a short period of time by reducing the number of scanning operations, it is effective to increase a region irradiated with thermal plasma. For this reason, a technique for generating thermal plasma within a large area has long been examined.
For example, a method is disclosed in which, in a plasma jet ejected from an external nozzle of a plasma torch, width-widening gases for use in width-widening the plasma jet in a direction intersecting with the center axis of the external nozzle are ejected simultaneously from two portions so that the plasma jet is widened (for example, see Japanese Unexamined Patent Publication No. 08-118027). Alternatively, another, method is proposed in which a plasma nozzle, which is characterized in that its mouth of the nozzle passage is tilted with a predetermined angle relative to the axis center of the nozzle passage, is prepared, and a casing that forms the nozzle passage, or one portion of such a casing, is rotated at a high speed around the longitudinal axis center so that the plasma nozzle is allowed to pass and shift along a work piece (for example, see Japanese Unexamined Patent Publication No. 2001-68298). Moreover, a system is disclosed in which a rotation head having at least one plasma nozzle disposed in an eccentric manner is provided (for example, see Japanese Unexamined Patent Publication No. 2002-500818).
Additionally, although not intended to process a large area in a short period of time, a high-speed gas shield arc welding method is disclosed in which, as a welding method utilizing the thermal plasma, a band-shaped electrode is used, and a welding process is carried out with its width direction being coincident with the welding line direction (for example, see Japanese Unexamined Patent Publication No. 04-284974).
Moreover, there is disclosed an inductive coupling-type plasma torch having a linear elongated shape, which uses an insulator member having a flat rectangular parallel epiped shape (for example, see Japanese Unexamined Patent Publication No. 2009-545165).
Additionally, a method for generating elongated linear plasma by using an elongated electrode is proposed (for example, see Japanese Unexamined Patent Publication No. 2007-287454). Although this system is described as a method for generating thermal plasma, this system relates to generation of low-temperature plasma, and is not suitable for thermal treatment. Supposing that this is used for generating thermal plasma, the arc discharge is concentrated on one portion because this system is a capacitive coupling-type using electrodes, and it is considered to be difficult to generate thermal plasma that is uniform in the longitudinal direction. In contrast, as a low-temperature plasma processing apparatus, this system provides a device capable of carrying out plasma processing, such as etching or film-forming, by forming an etching gas or a CVD (Chemical Vapor Deposition) gas into plasma.
Moreover, a system is disclosed in which a linear elongated plasma torch is formed by aligning a plurality of discharging electrodes into a line pattern (for example, see Japanese Unexamined Patent Publication No. 2009-158251).
However, the conventional technique for generating thermal plasma on a large area is not effectively applied to high-temperature treatment on the proximity of a surface of a substrate only in a very short period of time, such as a crystallizing process of a semiconductor.
In the technique for generating thermal plasma on a large area, described in Japanese Unexamined Patent Publication No. 08-118027 shown in the conventional examples, although the region is widened, the temperature distribution in the widened region becomes 100° C. or more, making it impossible to realize uniform thermal treatment.
Moreover, in the techniques for generating thermal plasma on a large area described in Japanese Unexamined Patent Publication Nos. 2001-68298 and 2002-500818 shown in the conventional examples, since the thermal plasma is inherently rocked, the period of time during which the thermal treatment is actually carried out becomes shorter than that of the scanning process without rotation, with the result that the period of time during which the large area is processed in not particularly shortened. Moreover, in order to carry out a uniform process, the rotation speed needs to be sufficiently made higher as compared with the scanning speed, inevitably causing a complicated structure of a nozzle.
Moreover, the technique described in Japanese Unexamined Patent Publication No. 04-284974 shown in the conventional examples is a welding technique, and is not a structure for processing a large area uniformly. Even in an attempt to apply this technique to the processing for a large area, since the dot-shaped arc oscillates along the belt-shaped electrode in this structure, nonuniform plasma is instantaneously caused although the plasma is uniformly generated on a time-averaged basis. Therefore, this technique is not applicable to the uniform processing for a large area.
Furthermore, the technique described in Japanese Unexamined Patent Publication No. 2009-545165 shown in the conventional examples is different from the structure using a DC arc discharge disclosed in “Crystallization of Si in Millisecond Time Domain. Induced by Thermal Jet Irradiation” S. Higashi, H. Kaku, T. Okada, H. Murakami, and S. Miyazaki, Japanese Journal of Applied Physics, Vol. 45, No. 5B, (2006) pp. 4313-4320 and Japanese Unexamined Patent Publication No. 2008-53634, and is characterized in that an inductive coupling-type high-frequency plasma torch is utilized. Since this is an electrodeless discharging process, there is an advantage that thermal plasma with superior stability is obtained (with small time-based fluctuations) and the electrode materials are hardly mixed into the substrate (contamination).
In the inductive coupling-type plasma torch, in general, a method has been adopted in which, in order to protect the insulator material from high-temperature plasma, the insulator material is formed into a double-tube structure with a coolant being allowed to flow therebetween. However, in the technique described in Japanese Unexamined Patent Publication No. 2009-545165 shown in the conventional examples, since an insulator material has a flat rectangular parallelepiped shape, it is not possible to flow a coolant with a sufficient flow rate by simply forming the insulator material into a double-tube structure. The reason for this is because, since the insulator material is generally inferior in mechanical strength to metals, the inner pressure in the double tubes cannot be made higher if the insulator material is made too long in the longitudinal direction. Consequently, it has limitations in processing a large area uniformly.
Although there is assumed to be no issue with cooling in the insulator material, in the technique described in Japanese Unexamined Patent Publication No. 2009-545165 shown in the conventional examples, since the high-temperature plasma formed in the inner space of the insulator material is such that only one portion thereof ejected from its lowermost portion, is directly reacted with the substrate, there is an issue in that power efficiency is poor. Moreover, in the inner space of the insulator material, since the plasma density near the center becomes higher, the plasma becomes nonuniform in the longitudinal direction, resulting in an issue in which the substrate is not processed uniformly.
Additionally, even in the case of a dot-shaped thermal plasma, since the number of scanning times can be reduced upon processing a large area if the diameter of the thermal plasma is large, a process in a short period of time can be achieved depending on applications. However, when the diameter of the thermal plasma is large, since the time during which the thermal plasma passes over the substrate upon scanning becomes substantially longer, it is not possible to carry out a high-temperature process only on the proximity of the surface of the substrate only for a short period of time. Therefore, a rather deeper region in the substrate becomes a high temperature to sometimes cause issues such as, for example, cracks in the glass substrate and film separations.
Moreover, in the technique described in Japanese Unexamined Patent Publication No. 2009-158251 shown in the conventional examples, as compared with the above-mentioned inductive coupling-type high-frequency plasma torch, there are disadvantages in that stability in thermal plasma is poor (time-based fluctuations are large) and the electrode materials tend to be mixed into the substrate (contamination).